Added, thanks, Ollie ... but with a proviso. I was brought up on the Southern Electric part of
BR▸ , where headcodes were not the train reporting numbers but rather the route numbers - goodness, I can still recall from my youth, standing on Petts Wood station and waiting for the London trains:
12 Orpington - Charing Cross
13 Orpington - Cannon Street
16 Sevenoaks - Charing Cross
17 Sevenoaks - Cannon Street
70 Orpington - Victoria
71 Orpington - Holborn Viaduct
73 Orpington - Holborn Viaduct via the Catford Loop
Then there were 4, 5, 18 and 90 ... and 22 and 33 on the Hastings line, those last two numbers allocated to that service when it went from steam to Diesel Electric - they had been left out of earlier numbering systems since each train had - prior to that point - carried one set of metal cut out digits, so double numbers (same digit twice) had been impossible ...
P.S. Updates now at
http://www.firstgreatwestern.info/acronyms.htmlto give it a permanent URL and to avoid this thread getting very long on each page. Over 250 terms / acronyms / abbreviations!